how to show radio button selected default in html code example

Example 1: radio by default selected

checked="checked"

Example 2: clear the input when change the radio button

$('input[name="am_payment"]').on('click', function() {
   if ($(this).val() === '') {
      $('#theamount').val('').prop("removed", true).focus();
   }
   else {
      $('#theamount').val('').prop("disabled", false);
   }
});